lib/bitmap: fix off-by-one in bitmap_to_arr64()

GENMASK*() family takes the first and the last bits of the mask
*including* them. So, with the current code bitmap_to_arr64()
doesn't clear the tail properly:

nbits %  exp             mask                must be
1        GENMASK(1, 0)   0x3                 0x1
...
63       GENMASK(63, 0)  0xffffffffffffffff  0x7fffffffffffffff

This was found by making the function always available instead of
32-bit BE systems only (for reusing in some new functionality).
Turn the number of bits into the last bit set by subtracting 1.
@nbits is already checked to be positive beforehand.

Fixes: 0a97953fd2 ("lib: add bitmap_{from,to}_arr64")
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>

lib/bitmap.c

@@ -1564,7 +1564,7 @@ void bitmap_to_arr64(u64 *buf, const unsigned long *bitmap, unsigned int nbits)
 
 	/* Clear tail bits in the last element of array beyond nbits. */
 	if (nbits % 64)
-		buf[-1] &= GENMASK_ULL(nbits % 64, 0);
+		buf[-1] &= GENMASK_ULL((nbits - 1) % 64, 0);
 }
 EXPORT_SYMBOL(bitmap_to_arr64);
 #endif